1. Field of the Invention
This invention relates to a welding accessory, particularly to a welding accessory for use during welding to prevent the outflow of molten metal and mold the same into a fixed shape, and to shield the weld portion from external air.
2. Description of the Prior Art
In arc stud welding, a welding accessory called an arc shield (ferrule) has conventionally been used for molding the shape of the molten metal, shielding the weld portion from external air, and other such purposes.
Such arc shields have in the past been made of, for example, ceramic. The conventional ceramic arc shield breaks under the thermal shock (1,300.degree. C. and higher) during welding and cannot be reused. Therefore after welding it is removed from the weld portion by smashing it with a hammer or the like.
Thus the conventional ceramic arc shield entails the problems that it is an expendable item which must be employed one for each stud and cannot be reused and, moreover, that it must be fitted one per stud and then, after welding, be smashed for removal.
In order to overcome these problems, it has been proposed for example in Japanese Unexamined Patent Publication No. 59(1984)-141379 and Japanese Unexamined Utility Model Publication No. 60(1985)-181269 that the arc shield be made reusable a number of times by constituting it from a material with superior thermal shock resistance property and forming it of separable segments.
However, the aforesaid Japanese Unexamined Utility Model Publication No. 60(1985)-181269 merely mentions ceramic as a material with superior thermal shock resistance property and does not disclose any specific material which, having higher thermal shock resistance than the conventional ceramic arc shield, is capable of withstanding numerous weldings (thermal shocks).
Further, while the aforesaid Japanese Unexamined Patent Publication No. 59(1984)-141379 discloses silicon nitride ceramic as a material having superior thermal shock resistance property, experiments conducted by the present inventors show that this silicon nitride ceramic is broken by thermal shock after one or two stud weldings and does not have adequately superior thermal shock resistance property to enable numerous repeated uses.